This invention relates generally to ultrasonic transducers for liquid level gauges, and more particularly to an ultrasonic transducer suitable for use in corrosive environments.
Ultrasonic liquid level measurement is fundamentally based on the emission of ultrasonic energy by a transmitter at a measuring station and the reflection of the transmitted energy from the surface of the liquid whose level is to be gauged to a receiver at the same station. Transmitted ultrasonic pulses are directed at the surface of the liquid through the vapor or gas thereabove. The elapsed transit time from the transmitter to the receiver is measured electronically and computed to provide an accurate measure of the distance between the measuring station and the liquid surface, thereby providing a reading of liquid level.
The ultrasonic measuring gauge is especially adapted to measure liquid level in mine shafts and oil wells, where in many instances the environment above the liquid level is corrosive in nature. In conventional forms of ultrasonic liquid-level measuring gauges, the transducer includes an electrostriction element having thin-film electrodes made of a conductive metal, such as silver, formed on its faces so as to produce homogeneous electrostriction. When an element of this type is subjected to a corrosive gas, it is eroded thereby and ultimately rendered inoperative.
In ultrasonic transducers of the type heretofore employed, such as that disclosed in Japanese Utility Model Ser. No. 37192/1974, the electrostriction element is disc-shaped and is affixed to a rigid solid body of greater thickness. This body serves to reduce the inherent resonance frequency of the electrostriction element to an extent affording a resonance frequency in a range suitable for liquid level measurement. The element and rigid body affixed thereto are supported within a case by an ordinary flange secured to a nodal point on the body.
Such known types of ultrasonic transducers have serious practical deficiencies, quite apart from the fact that they are subject to corrosion. Because of the flange arrangement, a considerable amount of ultrasonic energy is dissipated radially through the flange, this leakage markedly reducing the efficiency of the transducer.
Moreover, with existing types of ultrasonic transducers, the same transducer cannot always be used both as a transmitter and a receiver, in that when functioning to measure short distances, the vibrations of the transducer at the termination of transmission are not arrested or damped with sufficient rapidity to render the same transducer operative as a receiver at the instant the echo pulse reflected from the liquid surface arrives. It has, therefore, heretofore been necessary to provide separate transducers for transmission and reception for measuring short distances.